In order to fabricate integrated circuits, the structures formed on a mask are transferred, while being projected onto wafers covered with a photosensitive layer. After carrying out a developer step, the image structures formed in the layer on the wafer are transferred, in a further etching step, into an underlying layer that is to be patterned. Conventionally, masks, in which opaque or semitransparent structures are formed on an essentially transparent quartz substrate or similar materials, are used in production. For the projection, radiation is sent through the masks, so that the areas not covered by opaque structures are imaged in the photosensitive layer.
Technologies that are in development and will be used in the near future provide for the use of reflection masks, in particular those which can be used in the extreme ultraviolet (EUV) wavelength range of between 10 and 14 nm exposure wavelength. The reason is that the materials that can be used for mask substrates are no longer transparent in these short wave wavelength ranges. The reflection masks therefore comprise, on the surface of the substrate a reflective layer, typically a layer stack, with an alternating layer sequence of molybdenum and silicon, a buffer layer and also an absorber layer arranged thereon. The reflection masks are irradiated at an angle of incidence that is not perpendicular to the surface, so that the reflection layers uncovered by the removal of absorber layer areas are imaged as structures via a lens system on the wafer.
In the case of reflection masks as well as transmission masks, it is crucial for the imaging quality that the distribution of the reflectivity or transmission over the mask surface is as homogeneous as possible. A locally reduced reflectivity or transmission leads to a weaker through exposure and thus, if appropriate, to a smaller line width of an exposed structure element, at the corresponding location in the photosensitive layer on the wafer. Particularly, in the case of EUV reflection masks, a locally varying reflectivity may have a direct influence on the process window during the imaging onto the wafer. It is necessary, therefore, to solve the problem of a locally varying reflectivity, as well as transmission, as early in the fabrication of the mask or in the development of the fabrication process in order to afford possibilities for avoidance.
It is possible to use fully reflective or transmissive mask substrates where the opaque or absorber layers are completely removed in a test exposure and to evaluate the intensity distribution arising in the image plane of the wafer. As an alternative, it is also possible to measure the reflectivity or transmission at larger open areas of the mask within which no opaque or absorbing structures are provided.
However, variations of the reflectivity or transmission in large, open areas may be very different from those within patterned regions. However, the process window of an imaging is precisely influenced, on the other hand, also by line width fluctuations within the patterned regions. If, moreover, the reflectivity or transmission is also governed by the density of the structures on the mask, then satisfactory conclusions about the influence of reflection or transmission fluctuations are virtually precluded.
The varying etching depth on the mask, for example, being dependent, if appropriate, on the width of a gap in the resist that has been exposed by an electron beam, beforehand in order to form the relevant structure on the mask. A narrower gap width may lead to a lower etching rate and thus to a smaller etching depth. If the opaque or absorber material is to be removed, then residues of the corresponding material may possibly remain on the reflective or transparent layer (substrate) and thus locally influence the reflection or transmission behavior.
Further examples relate to a silicon layer that is locally oxidized during etching, for instance, and is thus subject to increasingly greater absorption, or the formation of thin carbon films on the substrates on account of contamination.
Thus, conclusions about the causes of a line width variation of the image arising on the wafer can be obtained only to a very limited extent by means of conventional measurements of the local variations of reflectivity or transmission. Conversely, if, on account of ascertained line width fluctuations on the wafer, detailed measurements of the line widths of the mask are carried out in a microscope, but the ascertained gap widths exhibit nothing conspicuous, then it is also possible, with difficulty however, to draw conclusions about a varying reflection or transmission behavior as a cause of the fluctuations.